Thulium-doped germanate glass composition and device for optical amplification

ABSTRACT

A Tm-doped germanate glass composition comprises GeO 2  having a concentration of at least 20 mole percent, Tm 2 O 3  having a concentration of about 0.001 mole percent to about 2 mole percent, and Ga 2 O 3 , having a concentration of about 2 mole percent to about 40 mole percent. The composition can further include an alkaline earth metal compound selected from the group consisting of MgO, CaO, SrO, BaO, BaF 2 , MgF 2 , CaF 2 , SrF 2 , BaCl 2 , MgCl 2 , CaCl 2 , SrCl 2 , BaBr 2 , MgBr 2 , CaBr 2 , SrBr 2 , and combinations thereof, and having a non-zero concentration of less than about 40 mole percent. The composition can further include an alkali metal compound selected from the group consisting of Li 2 O, Na 2 O, K 2 O, Rb 2 O, Cs 2 O, Li 2 F 2 , Na 2 F 2 , K 2 F 2 , Rb 2 F 2 , Cs 2 F 2 , Li 2 Cl 2 , Na 2 Cl 2 , K 2 Cl 2 , Rb 2 Cl 2 , Cs 2 Cl 2 , Li 2 Br 2 , Na 2 Br 2 , K 2 Br 2 , Rb 2 Br 2 , Cs 2 Br 2  and combinations thereof, and having a non-zero concentration of less than about 20 mole percent. The emission bandwidth of the composition in the 1450 nm to 1530 nm range can be varied on the basis of one or more composition ratios. A Tm-doped germanate optical amplifier can amplify optical signals in the S-band region of the electromagnetic spectrum.

BACKGROUND OF THE INVENTION

[0001] 1. Field of The Invention

[0002] This invention generally relates to the field of glasscompositions and devices for optical amplification.

[0003] 2. Description of the Related Art

[0004] In optical telecommunications networks, high bandwidth is desiredfor applications such as the Internet, video on demand, and videophone.In many optical communications systems, optical signals havingwavelengths in the range 1530-1560 nanometers (nm) are utilized. Thiswavelength range corresponds to the “C-band” in telecommunications. Thiswavelength range also corresponds to a minimum attenuation region forsilica and silica-based fibers.

[0005] Optical amplifiers are utilized to amplify the optical signals inthose wavelength regions. Conventional optical amplifiers fortelecommunications include erbium (Er)-doped silicate glass. TheEr-doped silicate glass optical amplifier operates in the C-band and canalso amplify optical signals in the 1570 nm-1620 nm range (also referredto as the L-band).

[0006] The ever-increasing demand for bandwidth has filled the erbiumC-band, and is beginning to fill the L-band. In order to increaseoptical bandwidth, more wavelengths will need to be transmitted. Onewavelength range of interest is the 1460 nm-1530 nm wavelength band,often referred to as the “S-band.” However, this wavelength band isoutside of the Er-based material amplification range.

[0007] Within the 1460 nm-1530 nm wavelength band, trivalent thulium(Tm³⁺) has an emission band centered at about 1470 nm. As shown in theTm³⁺ energy diagram of FIG. 1, the ³H₄-³F₄ transition in Tm³⁺corresponds to an emission at about 1470 nm. In order to generate apopulation in the ³H₄ energy level, for example, 790 nm radiation isabsorbed by the Tm³⁺ material, whereby ions are transferred to the ³H₄excited state from the ³H₆ ground state.

[0008] Most Tm-doped silicate glasses have an excited state lifetime(for the ³H₄ level) of less than 100 microseconds, due to the quenchingof the upper level in silicate hosts. This short lifetime is lesspreferable for laser and amplification applications. Similarly, otherTm³⁺ hosts, such as phosphate glass and borate glass, are also lesspreferable because Tm³⁺ is quenched by the high phonon energy of theseglasses as well.

[0009] An increased ³H₄ excited state lifetime can be obtained with aTm-doped host fluoride glass material, such as fluorozirconate or ZBLAN(57ZrF₄-20BaF₂-4LaF₃-3AlF₃-20NaF). The measured lifetime for the ³H₄excited state lifetime in ZBLAN is about 1.5 milliseconds. While laseraction and optical amplification have been previously demonstrated inTm-doped ZBLAN, this material is not advantageous for mass-producedoptical amplifier applications because of the difficulties of processingfluoride glasses, the low glass transition temperature, and the lessthan desirable chemical durability of fluoride glasses, which sufferfrom deleterious effects when exposed to moisture. In addition, theemission linewidth in ZBLAN is narrow, limiting the bandwith of theamplifier.

[0010] Thus, there remains a need for optical amplifiers that operate inthe 1460 nm-1530 nm wavelength band.

SUMMARY OF THE INVENTION

[0011] In view of the foregoing, according to one embodiment of thepresent invention, a composition comprises GeO₂ having a concentrationof at least 20 mole percent, Tm₂O₃ having a concentration of about 0.001mole percent to about 2 mole percent, and Ga₂O₃, having a concentrationof about 2 mole percent to about 40 mole percent. The composition canfurther include an alkaline earth metal compound selected from the groupconsisting of MgO, CaO, SrO, BaO, BaF₂, MgF₂, CaF₂, SrF₂, BaCl₂, MgCl₂,CaCl₂, SrCl₂, BaBr₂, MgBr₂, CaBr₂, SrBr₂, and combinations thereof, andhaving a non-zero concentration of less than about 40 mole percent. Thecomposition can further include an alkali metal compound selected fromthe group consisting of Li₂O, Na₂O, K₂O, Rb₂O, Cs₂O, Li₂F₂, Na₂F₂, K₂F₂,Rb₂F₂, Cs₂F₂, Li₂Cl₂, Na₂Cl₂, K₂Cl₂, Rb₂Cl₂, Cs₂Cl₂, Li₂Br₂, Na₂Br₂,K₂Br₂, Rb₂Br₂, Cs₂Br₂ and combinations thereof, and having a non-zeroconcentration of less than about 20 mole percent. The emission bandwidthand lineshape of the composition in the 1450 nm to 1530 nm range can bevaried on the basis of one or more composition ratios and/or otherparameters.

[0012] According to another embodiment of the present invention, anoptical amplification device comprises a germanate glass material dopedwith Tm³⁺. The germanate glass material has a first surface configuredto receive an optical signal having a wavelength of from about 1460 nmto about 1540 nm and a second surface configured to output an amplifiedoptical signal. The germanate glass material can have the compositiondescribed above. The emission bandwidth of the germanate glass materialcan be varied based on the composition of the material. The germanateglass material can be configured as a core for an optical fiber. Theoptical amplification device can further include a pump source forproducing an excited ³H₄ state in Tm³⁺.

[0013] Other advantages and novel features of the present invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The accompanying drawings, which are incorporated herein and formpart of the specification, illustrate the present invention and,together with the description, further serve to explain the principlesof the invention and to enable a person skilled in the pertinent art tomake and use the invention.

[0015]FIG. 1 shows an energy level diagram for Tm³⁺.

[0016]FIG. 2 shows a plot of the emission spectra for several Tm-dopedgermanate glasses having varying concentrations of Ga₂O₃ in mole %.

[0017]FIG. 3 shows a plot of the lifetime of the Tm³⁺ ³H₄ and ³F₄ levelsas a function of Tm₂O₃ concentration.

[0018]FIG. 4 shows a plot of the lifetime of the Tm³⁺ ³H₄ upper level asa function of β-OH.

[0019]FIG. 5 shows an emission spectra for several Tm-doped germanateglass material samples according to an embodiment of the invention.

[0020]FIG. 6 shows an emission spectra for a Tm-doped germanate glassmaterial samples according to another embodiment of the invention.

[0021]FIG. 7 shows a Tm-doped germanate glass material optical amplifieraccording to another embodiment of the invention.

[0022]FIGS. 8A and 8B show Tm-doped germanate glass fiber according toan alternative embodiment of the invention.

[0023]FIG. 9 shows a plot of index of refraction versus modifierconcentration for several different modifiers to the germanatecomposition.

[0024]FIG. 10 shows gain curves over the infrared wavelength region fortwo germanate compositions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] The present invention is related to a Tm-doped germanate glasscomposition that can be utilized as a wide band optical amplifier in the1460 nm-1530 nm wavelength band.

[0026] The inventors have determined that germanate glasses can act as ahost material for Tm³⁺. The Tm-doped germanate glass material hasacceptable low phonon energy characteristics, good chemical durability,and can be processed in a straightforward manner. In addition, theinventors have determined that the 1460 nm-1530 nm wavelength emissionshape can be both controlled and maximized in particular wavelengthregions depending on the specific composition of the germanate glass.

[0027] According to a first embodiment of the present invention, a hostmaterial for Tm-doping is provided. The host material is agermanate-based glass having a GeO₂ glass former and an oxide compoundsuch as Ga₂O₃, or the like, that can provide added chemical durability.Ga₂O₃ is preferable in a concentration of about 2 mole % to about 40mole %, with a concentration of about 10 mole % to about 18 mole % beingpreferred. By “about” it is meant within + or −1.0%.

[0028] The Tm³⁺ ion can be introduced into the germanate glasscomposition as a particular concentration of Tm₂O₃ (or a thulium halide,such as TmF₃, TmCl₃, and the like). The Tm³⁺ dopant concentration can befrom about 0.001 mole % to about 2 mole %, with a preferredconcentration being from about 0.05 mole % to about 0.1 mole %. Forexample, acceptable results can occur for Tm³⁺ concentrations of about0.05 mole % (i.e., 0.16 weight %). Acceptable results can also occur forTm³⁺ concentrations of about 0.5 weight %.

[0029] One or more of the following groups of materials can also beincorporated into the germanate host material. First, alkaline earthmetal compounds can be included. For example, the alkaline earth metalcompounds can include BaO, MgO, CaO, SrO, and/or BaF₂, MgF₂, CaF₂, SrF₂and/or BaCl₂, MgCl₂, CaCl₂, SrCl₂, and/or BaBr₂, MgBr₂, CaBr₂, SrBr₂, orcombinations thereof. The alkaline earth compounds can help providebetter chemical durability and glass stability for the germanate host.In addition, the alkaline earth fluorides can help increase the lifetimeof the Tm³⁺ emission, which can increase the efficiency of opticalamplification. Further, alkaline earth metal halides such as, forexample, BaCl₂, CaCl₂ and CaBr₂ can act as drying agents to strip out—OH during the melting process. For use herein, AO is a general alkalineearth metal oxide, and AX₂ is a general alkaline earth metal halide.

[0030] Second, alkali oxide compounds can be included in the germanatehost. These alkali oxide compounds can include, for example, Li₂O, Na₂O,K₂O, Rb₂O, Cs₂O, Li₂F₂, Na₂F₂, K₂F₂, Rb₂F₂, Cs₂F₂, Li₂Cl₂, Na₂Cl₂,K₂Cl₂, Rb₂Cl₂, Cs₂Cl₂, Li₂Br₂, Na₂Br₂, K₂Br₂, Rb₂Br_(2,) and Cs₂Br₂, andcombinations thereof. One or more of these compounds can be incorporatedto provide glass melting stability and gain shaping. Further, alkalimetal halides such as, for example, Na₂F₂, K₂Cl₂, and Rb₂Br₂ can act asdrying agents to strip out —OH during the melting process. For useherein, Z₂O is a general alkali metal oxide, and Z₂X₂ is a generalalkali metal halide. The halides are represented as Z₂X₂ so that theyhave the same stoichiometry as the oxide. The person of skill in the artwill recognize that Z₂X₂ is chemically identical to ZX; for example,Na₂Cl₂ is sodium chloride.

[0031] Third, intermediate elements and compounds (e.g., Ta₂O₅, La₂O₃,Nb₂O₅, Al₂O₃, Sb₂O₃, As₂O₃, and CeO₂) can be included. For example,Ta₂O₅ and Nb₂O₅ can be incorporated in sufficient amounts to increasethe index of refraction of the germanate host material. Al₂O₃ can beincorporated in suitable amounts to increase durability, and/or todecrease the germanate host index of refraction, if used in appropriateamounts. Sb₂O₃, As₂O₃, and/or CeO₂ can be incorporated in the germanateglass host as refining agents to help remove bubbles from the glassduring formation. They can also be used to control the oxidation stateof polyvalent compounds in the glass.

[0032] Fourth, heavy metal oxides and compounds (e.g., PbO and/or Bi₂O₃)can be incorporated into the germanate host material. One or more ofthese compounds can be incorporated into the composition to provideincreased glass stability, a modified refractive index, and/or gainshaping.

[0033] Other materials such as RE₂O₃ (where RE represents all rare earthelements, such as La, Nd, Pr, Er, Y, Yb, Er, Sm, Gd, Lu, etc.), ZnO,TiO₂, ZrO₂, and HfO₂ can be incorporated in germanate host for variousother purposes. For example, La₂O₃, Gd₂O₃, Y₂O₃, and Lu₂O₃ aretransparent at the wavelength of interest and can help reduce Tm³⁺clustering. Also, compounds such as La₂O₃, Ta₂O₃, PbO, and HfO₂, asshown in FIG. 9, as well as other compounds such as Al₂O₃ and similarmaterials can be incorporated in differing amounts, depending on thedesired index of refraction of the germanate material to controlrefractive index without modifying the Tm³⁺ spectroscopy, which can becontrolled by compositional parameters, such as R₁ (defined below). Withthese particular modifier compounds, increased concentrations can leadto an increased (or in some cases, decreased) index of refraction. Asthe person of skill in the art will appreciate, these compounds may alsobe incorporated as the corresponding halide.

[0034] In particular, the germanate glass material of a preferredembodiment can include one or more variations of the followingcomposition: GeO₂+Ga₂O₃+Tm₂O₃+(AO or AX₂) and/or (Z₂O orZ₂X₂)+(optionally) one or more intermediate compounds, where A can be analkaline earth metal such as Mg, Ca, Sr, or Ba, and Z can be an alkalimetal such as Li, Na, K, Rb, or Cs. This composition provides a lowphonon energy host for the Tm³⁺ ion. In addition, chemical durabilityand gain shape can be improved over a fluoride-based material such asZBLAN. Moreover, the lifetime of the ³H₄-³F₄ transition can be over 5times greater than that demonstrated in Tm-doped silicate materials.

[0035] Table I shows a first preferred range of concentrations of theaforementioned germanate glass composition constituents. TABLE IComposition Concentrations Concentration Ranges, including preferableand Component preferred ranges (in mole percent) Tm₂O₃ 0.001% ≦ Tm₂O₃ ≦2% (preferable), 0.05% ≦ Tm₂O₃ ≦ 0.1% (preferred) GeO₂ GeO₂ ≦ 20%, 50% ≦GeO₂ ≦ 90% (preferable), 65% ≦ GeO₂ ≦ 75% (preferred) SiO₂ SiO₂ ≦ 40%SiO₂ + GeO₂ 40% ≦ SiO₂ + GeO2 ≦ 80% ZnO 0% ≦ ZnO ≦ 40% (preferable), 0%≦ ZnO ≦ 5% (preferred) PbO 0% ≦ PbO ≦ 50% (preferable), 0% ≦ PbO ≦ 10%(preferred) Bi₂O₃ 0% ≦ Bi₂O₃ ≦ 50% PbO + Bi₂O₃ 0% ≦ PbO + Bi₂O₃ ≦ 60%Ga₂O₃ Ga₂O₃ ≧ 2%, 2% ≦ Ga₂O₃ ≦ 40% (preferable), 10% ≦ Ga₂O₃ ≦ 18%(preferred) Al₂O₃ 0% ≦ Al₂O₃ < 20% (preferable), 0% ≦ Al₂O₃ < 1%(preferred) Ta₂O₅, Nb₂O₅ 0% ≦ Ta₂O₅ ≦ 20%, 0% ≦ Nb₂O₅ ≦ 10%(preferable), 0% ≦ Ta₂O₅ ≦ 2% (preferred), 0% ≦ Nb₂O₅ (preferred) Sb₂O₃0% ≦ Sb₂O₃ ≦ 5% (preferable), 0% ≦ Sb₂O₃ ≦ 1% (preferred) As₂O₃ 0% ≦As₂O₃ ≦ 5% (preferable), 0% ≦ As₂O₃ ≦ 1% (preferred) CeO₂ 0% ≦ CeO₂ ≦ 5%(preferable), 0% ≦ CeO₂ ≦ 2% (preferred) RE₂O₃ 0% ≦ RE₂O₃ ≦ 15%(preferable), 0% ≦ RE₂O₃ ≦ 1% (preferred) AX₂ + Z₂X₂ 0% ≦ AX₂ + Z₂X₂ ≦10% (preferable), 0.1% ≦ AX₂ + Z₂X₂ ≦ 4% (preferred) TiO₂ + ZrO₂ + 0% ≦TiO₂ + ZrO₂ + HfO₂ ≦ 10% (preferable), HfO₂ 0% ≦ TiO₂ + ZrO₂ + HfO₂(preferred) AO + AX₂ 0% ≦ AO + AX₂ ≦ 40% (preferable), 7%-14% (preferredAO + AX₂), 0-1% (preferred MgO), 4%-10% (preferred CaO), 0-5% (preferredSrO), 2%-7% (preferred BaO) Z₂O + Z₂X₂ 0% ≦ AO + AX₂ ≦ 20% (preferable),4%-6% (preferred AO + AX₂), 0% (preferred Li₂O), 0% (preferred Na₂O),2%-6% (preferred K₂O), 2%-4% (preferred Rb₂O), 2%-4% (preferred Cs₂O)(AO + AX₂ + Z₂O + 0.4 ≦ R₁ ≦ 2.5 (no units) (preferable), Z₂X₂)/(Ga₂O₃ +0.8 ≦ R₁ ≦ 1.25 (no units) (preferred) Al₂O₃) = R₁ (AO + AX₂)/(Z₂O + Anyratio (preferable), Z₂X₂) = R₂ 1.8 ≦ R₂ ≦ 2.25 (no units) (preferred)

[0036] The above germanate composition can be varied according to anumber of parameters.

[0037] As the inventors have determined, pure GeO₂ has a very low Tm³⁺solubility (<0.25 mole %) and can lead to clustering and poor Tm³⁺efficiency. As described herein, alkali germanate glasses demonstrate abroad Tm³⁺ emission (150 nm FWHM), but the spectra can be extremelypeaked and the durability of the host glass is poor.

[0038] For example, FIG. 2 shows the emission spectra for severalTm-doped germanate glasses having varying concentrations of Ga₂O₃. Theemission spectra were taken under conventional techniques, where theTm-doped germanate sample was irradiated at about 800 nm, and emissionwas detected with an infrared detector. A ¼-meter monochromator was usedto provide spectral resolution.

[0039] In particular, the following germanate composition was used inFIG. 2: 60GeO₂+XXGa₂O₃+(40−XX)K₂O, where XX represents the mole % ofGa₂O₃ incorporated in the germante host material. In these spectra, 0.05mole % Tm₂O₃ was used. By adding Ga₂O₃ to an alkali or alkaline earthgermanate, the glass becomes more stable, more durable, and, as shown inFIG. 2, the emission spectrum becomes much flatter and more desirable.The glass stability and durability can also be further enhanced by usingan appropriate mixture of alkali and alkaline earth modifiers. Thepreferred germanate glass compositions have an alkaline earth metal toalkali metal ratio (R₂ from Table I) of about 2.0. Alkaline earthgermanates have good durability as compared to their alkalicounterparts, and these optimized mixtures are acceptable in comparisonto alkaline earth end members, and have improved glass stability forfiber drawing.

[0040] The Tm³⁺ concentration in the host material can also be varied.FIG. 3 shows that at concentrations above 0.1 mole %, the 1460 nm (i.e.,Tm³⁺ ³H₄ level) lifetime begins to decrease, indicating thatnon-radiative relaxation begins to occur at greater concentrations,which can decrease amplifier efficiency. The Tm³⁺ ³F₄ level is alsoquenched above 0.1% as shown in FIG. 3. The tradeoff to consider inoptimization is that it is desirable to put as much Tm₂O₃ in the glassas possible without sacrificing efficiency, so shorter fiber lengths canbe used to minimize the passive loss and noise figure of the amplifier.Thus, a preferred Tm₂O₃ concentration is between about 0.05 mole % toabout 0.1 mole %.

[0041] Another parameter that can be controlled is the water content (orβ-OH) of the germanate glass. OH ligands have a high frequencyvibrational mode that can couple to excited Tm³⁺ ions and quench them.FIG. 4 shows the effect of β-OH on the Tm³⁺ ³H₄ lifetime. Tm³⁺efficiency was maintained by having the glasses dried throughcompositional and processing innovations. For example, glass batches canbe calcined at 300° C. for 2 hours under dry flowing O₂ and then slowlyheated at 65° C./hr. under flowing O₂ to thermally devolve absorbedwater and flush it away before it has a chance to react with the batchmaterials, or get trapped in the melt. In this manner, most of the watercan be removed before any melt is formed. The addition of chlorides andfluorides to the batch help to remove OH as HCl and/or HF respectivelyat high temperatures after the melt has formed. Two mole % BaCl₂ and 2mole % CaF₂ were found to be preferred levels for effective drying. Thiscombination of processing and composition reduces the β-OH of the glassby a factor of 50 over conventional melting techniques and canconsequently increase the Tm³⁺ lifetime by about 30%, from 330 μs to 430μs. The addition of more fluoride can cause a further increase inlifetime (at constant β-OH) up to 520 μs, but the tradeoff offluorescence linewidth and glass stability should be considered.

[0042] Other parameters that can be varied to produce a desired emissionbandwidth are various composition ratios. As listed in Table 1, a firstcomposition ratio R₁ is defined as R₁=AX₂+Z₂O+Z₂X₂)/(Ga₂O₃+Al₂O₃). Thisratio can provide a measure of non-bonding oxygens in the glassmaterial.

[0043] A preferable range for R₁ is from about 0.4 to about 2.5, with apreferred range being from about 0.8 to about 1.25. For example, whenR₁=1, the glass network is fully polymerized and viscosity anddurability are maximized. As R₁ becomes greater than 1, the number ofnon-bonding oxygens increases, viscosity decreases and the emissionspectrum breaks up into many peaks, as illustrated in FIG. 2.

[0044] As listed in Table I, a second composition ratio is R₂, which isdefined as R₂=(AO+AX₂)/(Z₂O+Z₂X₂). Again, AF₂ can be substituted for AOand/or ZF for Z₂O, if desired. While any R₂ ratio is acceptable, a rangeof from about 1.8 to about 2.25 is preferred.

[0045] In addition, the above ratios can be modified when intermediateelements are used, in the manner apparent to those of skill in the art.

[0046] According to a preferred embodiment of the present invention, theemission bandwidth of the Tm³⁺ ³H₄-³F₄ transition can be maximizeddepending on the R₁ value and/or on the presence or absence of theforming metal oxide (such as PbO, Bi₂O₃) in the germanate compositionfor a given R₁ value.

[0047] For example, FIG. 5 shows a plot of the fluorescence (oremission) spectra for several different germanate glass compositionsformed in accordance with a composition of an embodiment of the presentinvention. In this example, R₁ values are set at R₁≧1 and the samplecompositions do not include any heavy metal compounds, such as PbO andBi₂O₃. Also shown in FIG. 2 is the transmission spectra for a Tm³⁺ ZBLANglass material as a comparison. The sample germanate glass compositionsused to produce the spectra shown in FIG. 5 are presented below in TableII. TABLE 2 Sample Compositions for Fig. 5 Concentra- Concentra-Concentra- Concentra- Constituent tion CA tion CB tion CC tion CDGeO₂(mole %) 60 60 60 60 Ga₂O₃ 8 12 16 20 (mole %) BaO(mole %) 30 26 2218 BaF₂(mole %) 2 2 2 2 Tm₂O₃(wt. %) 0.5 0.5 0.5 0.5 R₁ 4 2.333 1.5 1

[0048] In this example, R₁ is determined by the ratio (BaO+BaF₂)/(Ga₂O₃)in accordance with the definition provided previously. Alternatively,other intermediate compositions including compounds such as Al₂O₃ andTa₂O₅ and/or heavy metals such as PbO and Bi₂O₃ can be used. Inaddition, Tm³⁺ was introduced into each of the samples through a Tm₂O₃compound, having a concentration of about 0.5% (by weight). The emissionspectra were measured in the manner described above with respect to FIG.2.

[0049]FIG. 5 shows a substantial broadening in emission bandwidth (as afunction of wavelength) for the Tm³⁺ doped germanate compositions of thepresent invention. On the shorter wavelength side, for 1≦R₁≦4, a peakemission at about 1440 nm is shown. On the longer wavelength side, asecond peak at about 1525 nm is shown for 1.5≦R₁≦4. The spectra showthat for each of the germanate compositions, the emission band width isabout 140 nm-150 nm measured at full width at half maximum (FWHM). Theinventive glasses show a much stronger fluorescence in the desirable1480-1520 nm portion of the spectrum, relative to ZBLAN.

[0050] According to yet another embodiment of the present invention, theTm-doped germanate glass composition can include heavy metal glassforming oxides such as PbO and Bi₂O₃. For example, the germanate glasscomposition sample CX included: GeO₂ (45 mole %), SiO₂ (5 mole %), PbO(45 mole %), K₂O (10 mole %), and Tm₂O₃ (0.5 weight %). FIG. 6 shows aplot of the emission spectra for sample CX, as well as for a Tm³⁺ ZBLANglass material and a sample of glass composition CD as comparisons. Theemission spectra were measured in accordance with the experimentalconditions described previously with respect to FIG. 5. The results fromFIG. 6 show a similar broadening of the Tm³⁺ emission spectrum as wasshown in FIG. 2 as compared to the ZBLAN sample. In addition, FIG. 6shows a stronger peak for sample CE at 1525 nm as compared to sample CD,a reduced emission from about 1450 nm to about 1500 nm for CX ascompared to CD, and increased emission peaks at about 1440 nm and atabout 1400 nm for CX as compared to CD.

[0051] In addition to the above mentioned parameters, the amount ofheavy modifier elements (e.g., BaO) in the germanate composition can bevaried. Also, the amount of intermediate elements (e.g., Ta₂O₅, La₂O₃,and Ga₂O₃) in the germanate composition can be varied. Further, theamount of glass forming heavy metal oxides (e.g., PbO and Bi₂O₃) in thegermanate composition can be varied.

[0052] Overall, FIGS. 2-6 demonstrate that the concentrations of thecomponents of the germanate glass composition can be modified to providea tailored emission shape over a broad emission spectra extending fromabout 1400 nm to about 1540 nm. Thus, the germanate glass compositionscan be utilized for wide-band optical amplification in this extendedwavelength range.

[0053] In accordance with the Tm-doped germanate glass compositionsdescribed above, many variations of component concentrations were testedand samples produced. The following Tables III-IX list componentconcentrations for example glasses which are suitable for waveguideapplications, including slab amplifiers and fiber amplifiers. Thevariations listed include samples DD-OV. Sample OM is a preferredcomposition. The samples denoted by ** indicate unstable glass.

[0054] The glasses were made as follows. The constituent raw materialpowders (e.g., the oxides, chlorides, bromides, nitrates, and/orcarbonates) are weighed and mixed to form a batch for the desiredcomposition. As the person of skill in the art will appreciate, thealkali metal oxides and the alkaline earth metal oxides may be added asthe corresponding carbonates or nitrates. The batch can then be placedin a refractory crucible, such as SiO₂ (or, e.g., platinum, Al₂O₃, andthe like) and calcined at 300° C. to drive off physically absorbedwater. The batch is then further heated to a melting temperature of1350° C., to allow the batch materials to react and form a melt.Halogens (e.g., F, Cl, and Br) react with hydroxyl groups at thesetemperatures and form volatile HF, HCl, and/or HBr which further driesthe glass melt, and also strips out deleterious transition metals. Oncethe melt is formed, it can be transferred to a Pt crucible to preventsiliceous cord from the SiO₂ crucible. The melt can be stirred and thetemperature can be lowered to about 1150° C. to condition the melt forforming. Other methods of forming these glasses will be apparent tothose of ordinary skill in the art given the present description. TABLEIII Samples DD-DL Glass DD DE DF** DG DH DJ** DK DL GeO₂ 60 60 40 60 8060 60 60 K₂Cl₂ 2 0 0 0 0 0 2 2 K₂O 38 0 0 0 0 0 20 28 Cs₂O 0 40 0 0 0 00 0 PbO 0 0 58 38 0 0 0 0 PbCl₂ 0 0 2 2 0 0 0 0 Bi₂O₃ 0 0 0 0 20 0 0 0Nb₂O₅ 0 0 0 0 0 0 10 0 Ga₂O₃ 0 0 0 0 0 0 0 10 BaO 0 0 0 0 0 38 0 0 BaCl₂0 0 0 0 0 2 0 0 Tm₂O₃ 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

[0055] TABLE IV Samples DM-DU Glass DM DN DO** DP DQ DR DS DT DU GeO₂ 6060 40 60 60 60 60 60 60 K₂O 23 18 13 8 18 28 8 13 18 K₂Cl₂ 1 1 1 1 1 1 11 1 K₂Br₂ 1 1 1 1 1 1 1 1 1 Ga₂O₃ 15 20 25 0 0 0 10 10 10 BaO 0 0 0 3020 10 20 15 10 Tm₂O₃ 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

[0056] TABLE V Samples DV-EJ Glass DV DW DX DY DZ EA EB EC ED EE EF EGEH EI EJ** GeO₂ 60 60 60 50 70 55 55 60 60 60 60 60 60 60 60 K₂O 22 2016 23 13 8 13 12 12 K₂Cl₂ 1 1 1 1 1 1 1 1 1 K₂Br₂ 1 1 1 1 1 1 1 1 1Ga₂O₃ 16 18 22 25 15 0 0 16 16 16 18 22 18 18 22 PbO 0 0 0 0 0 15 10 100 24 22 16 BaO 0 0 0 0 0 10 10 0 0 24 22 16 ZnO 0 0 0 0 0 10 10 0 10Tm₂O₃ 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

[0057] TABLE VI Samples ID-IK Glass ID IE IF IG IH II IJ IK IM GeO₂ 7070 70 70 70 70 70 70 70 Ga₂O₃ 14 14 14 14 14 14 7 0 14 Al₂O₃ 0 0 0 0 0 07 14 0 BaO 0.67 0 4.67 1.56 2.67 2.67 2.67 2.67 0.67 BaCl₂ 2 0 2 2 2 2 22 2 CaO 8 10.7 0 3.56 5 5.5 6 6 8 MgO 0 0 4 3.55 0 0 0 0 0 ZnO 0 0 0 0 10.5 0 0 0 K₂O 2.67 2.67 2.67 2.67 2.67 2.67 2.67 2.67 5.33 Rb₂O 2.662.66 2.66 2.66 2.66 2.66 2.66 2.66 0 CeO₂ 0.25 0.25 0.25 0.25 0.25 0.250.25 0.25 0 Tm₂O₃ 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.05

[0058] TABLE VII Samples KX-LF Glass KX KY KZ LA LB LC LD LE LF GeO₂ 7070 70 70 70 70 70 70 70 Ga₂O₃ 14 14 14 14 14 14 14 14 14 BaO 2.67 2.672.67 2.67 2.67 2.67 2.67 2.67 2.67 CaF₂ 4 4 4 4 4 4 4 4 4 CaO 4 4 4 4 44 4 4 4 K₂O 2.67 2.67 2.67 2.67 2.67 2.67 2.67 2.67 2.67 Rb₂O 2.66 2.662.66 2.66 2.66 2.66 2.66 2.66 2.66 CeO₂ 0.05 0.05 0.05 0.05 0.05 0.010.25 0.5 1 Tm₂O₃ 0.01 0.05 0.1 0.2 0.4 0.05 0.05 0.05 0.05

[0059] TABLE VIII Samples HW-JM Glass HW JE JF JG JH JI JJ JK JL JM GeO₂70 70 70 70 70 70 70 70 70 70 Ga₂O₃ 14 14 14 14 14 14 14 14 14 14 Al₂O₃0 0 0 0 0 0 0 0 0 0 BaO 8.67 2.67 2.67 2.67 2.67 2.67 2.67 2.67 2.672.67 BaCl₂ 2 2 2 2 2 2 2 2 2 2 CaO 0 6 4 2 6 6 6 6 6 6 CaF₂ 0 0 0 0 0 00 0 0 0 K₂O 2.67 2.67 2.67 2.67 2.67 2.67 2.67 2.67 2.67 2.67 Rb₂O 2.662.66 2.66 2.66 2.66 2.66 2.66 2.66 2.66 2.66 CeO₂ 0.25 0.25 0.25 0.250.25 0.25 0.25 0.25 0.25 0.25 PbO 0 0 2 4 0 0 0 0 0 0 La₂O₃ 0 0 0 0 2 40 0 0 0 HfO₂ 0 0 0 0 0 0 2 4 0 0 Ta₂O₅ 0 0 0 0 0 0 0 0 2 4 Tm₂O₃ 0.1 0.10.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Index N/A 1.668 1.671 1.68 1.691 1.7281.667 1.671 1.688 1.719

[0060] TABLE IX Samples OM-OV Glass OM OR OS OT OU** OV** GeO₂ 70 70 7070 70 70 Ga₂O₃ 14 10 12 15 18 20 BaO 0 1.34 1.00 0.50 0 0 BaCl₂ 2.67 2 22 2 1.67 CaO 6 8 7 5.5 4.0 3.0 CaF₂ 2 2 2 2 2 2 K₂O 2.67 3.33 3.0 2.52.0 1.67 Rb₂O 2.66 3.33 3.0 2.5 2.0 1.66 Tm₂O₃ 0.05 0.05 0.05 0.05 0.050.05

[0061] According to another embodiment of the present invention, anoptical amplifier is provided. The optical amplifier includes a Tm-dopedgermanate glass host. The optical amplifier amplifies optical signalswithin the wavelength range of about 1450 nm to about 1530 nm,preferably about 1480 nm to about 1530 nm. For example, FIG. 10 showsgain curves as a function of wavelength for samples KL and KM. Here,gain is measured in dB/mW. Similar gain curves can be measured and usedto optimize the peak amplification wavelength region and length of theoptical amplifier.

[0062]FIG. 7 shows a schematic diagram of the optical amplifier 20. Aninput optical fiber 10 carries an optical signal 12 having a wavelengthof about 1450 nm to about 1530 nm. In this example, optical signal 12has a wavelength of about 1470 nm. Fiber 10 and output fiber 20 can beconventional silica-based optical fibers. The optical signal 12 isamplified by an optical amplifier 20. Optical amplifier 20 includes aninput surface 22 and an output surface 24. Optical amplification can beachieved over the wavelength range of interest using a Tm-dopedgermanate glass material, having a composition in accordance with theparameters described above. For example, optical amplifier 20 cancomprise a germanate glass material having the same composition assamples KY, OM, or CD, described previously, which exhibit a widebandemission at about 1450 nm to about 1530 nm. Alternatively, a germanateglass material having a different composition can be utilized, inaccordance with the parameters described above. The germanate glassmaterial can be in the form of a glass slab (as is shown in FIG. 7) withpolished input and output surfaces 22, 24 to reduce spuriousreflections. Alternatively, the germanate glass material can be used asthe core material of an optical fiber amplifier 50, as is shown in FIGS.8A and 8B. Input fiber 10 and output fiber 40 can be optically coupledto amplifier 20 using conventional fiber coupling techniques, as wouldbe apparent to one of skill in the art given the present description.

[0063] In addition, the optical amplifier 20 is optically pumped by apump source 30, such as a conventional laser diode (or other laser orlamp) operating in the 780 nm-800 nm wavelength region. Alternatively,an infrared Raman laser (or the like) can be used to pump both the³H₆-³F₄ and the ³F₄-³H₄ absorption bands simultaneously, in order toprevent possible deleterious up-conversion effects created when stronglypumping at 790 nm. Of course, any of a number of pumping schemes can beutilized so that the Tm-doped germanate glass material can efficientlyabsorb light from the pump source 30, depending on the particularabsorption characteristics of the Tm-doped germanate glass material, aswill be apparent to one of skill in the art given the presentdescription. Accordingly, at least a portion of the light from pumpsource 30 is absorbed by the Tm-doped germanate glass material inamplifier 20 in order to produce a population of Tm³⁺ ions in the ³H₄excited state. Optical amplification occurs through stimulated emissionas is known. As a result, the optical signal 12 is amplified and theamplified signal is output along output fiber 40. The length ofamplifier 20 and the Tm concentration can be modified depending on theoverall requirements of an optical system or network that amplifier 20is incorporated.

[0064] As mentioned previously, the Tm-doped germanate glass materialcan be incorporated into an amplifying fiber, such as fiber 50 as isshown in FIGS. 8A and 8B. In FIG. 8A, an input fiber 10 carrying anoptical signal is coupled by conventional techniques to amplifying fiber50, which is further coupled on the output to output fiber 40. A pump 30can be used to produce an excited state population in fiber 50 usingconventional fiber pumping techniques. As shown in FIG. 8B, amplifyingfiber 50 has a core 52 which includes the Tm-doped germanate glassmaterial described previously. The Tm-doped germanate glass material canbe drawn into fiber form using conventional fiber drawing techniques.Fiber 50 also includes a cladding material 54, which can include aninner clad and an outer clad. The cladding material should be suitablyindexed and can comprise a suitable conventional cladding material.Alternatively, the cladding material can be a borosilicate glass, asdescribed in the commonly held and copending U.S. patent application byDejneka et al. entitled “Borosilicate Cladding Glasses for GermanateCore Thulium-Doped Amplifiers,” which is incorporated herein byreference. An examplary borosilicate glass composition may be made froma composition having 45 mole % SiO₂, 5 mole % Al₂O₃, 13.5 mole % BaO, 2mole % BaCl₂, 5.5 mole % CaO, 2 mole % CaF₂, 7 mole % Na₂O, 20 mole %B₂O₃, and 1 mole % CeO₂. The cladding material may also be a germanateglass.

[0065] Thus, according to the preferred embodiments of the presentinvention, wide band optical amplification over the 1400 nm-1530 nmwavelength band can be achieved utilizing Tm-doped germanate glassmaterial.

[0066] It will be apparent to those of skill in the art that theinventive glasses may be used in applications other than Tm-dopedamplifiers or lasers. For example, the high germania content makes theundoped glasses useful for photorefractive applications such asoptically written gratings and waveguides. The material is alsowell-suited for use as a low phonon energy host for other rare earthions such as, for example, Yb³⁺, Er³⁺, Nd³⁺, and the like. Such dopedmaterials may be useful in applications such as amplifiers, lasers, andbroadband fluorescent light sources at wavelengths other than thoseacheivable with the Tm-doped glass.

[0067] Since each application has different requirements, compositionalmodifications may be made to the base glass for performanceoptimization. For example, SiO2 may be added to the base glass forapplications involving rare earth or transition metal ions not sensitiveto the phonon energy of the host. Likwise, B2O3 may be added to enhancethe photosensitivity of photorefractive materials.

[0068] While the above provides a full and complete disclosure of thepreferred embodiments of the present invention, various modifications,alternate constructions, and equivalents may be employed withoutdeparting from the scope of the invention. Therefore, the abovedescription and illustration should not be construed as limiting thescope of the invention, which is defined by the appended claims.

We claim:
 1. A composition, comprising: GeO₂ having a concentration ofat least 20 mole percent; Tm₂O₃ having a concentration of about 0.001mole percent to about 2 mole percent; and Ga₂O₃, having a concentrationof about 2 mole percent to about 40 mole percent.
 2. The compositionaccording to claim 1, further comprising: an alkaline earth metalcompound selected from the group consisting of MgO, CaO, SrO, BaO, BaF₂,MgF₂, CaF₂, SrF₂, BaCl₂, MgCl₂, CaCl₂, SrCl₂, BaBr₂, MgBr₂, CaBr₂,SrBr₂, and combinations thereof, and having a non-zero concentration ofless than about 40 mole percent.
 3. The composition according to claim2, further comprising: an alkali metal compound selected from the groupconsisting of Li₂O, Na₂O, K₂O, Rb₂O, Cs₂O, Li₂F₂, Na₂F₂, K₂F₂, Rb₂F₂,Cs₂F₂, Li₂Cl₂, Na₂Cl₂, K₂Cl₂, Rb₂Cl₂, Cs₂Cl₂, Li₂Br₂, Na₂Br₂, K₂Br₂,Rb₂Br₂, Cs₂Br₂ and combinations thereof, and having a non-zeroconcentration of less than about 20 mole percent.
 4. The compositionaccording to claim 3, wherein a composition ratio R₁ is greater than orequal to 0.4 and less than or equal to 2.5, wherein R₁ is defined by thefollowing relationship: R₁=(AO+AX₂+Z₂O+Z₂X₂)/(Ga₂O₃+Al₂O₃).
 5. Thecomposition according to claim 3, further comprising a third compoundselected from the group consisting Ta₂O₅, having a concentration of lessthan about 15 mole percent, Bi₂O₃, having a concentration of less thanabout 50 mole percent, Al₂O₃, having a concentration of less than about20 mole percent, PbO, having a concentration of less than about 50 molepercent, and combinations thereof.
 6. The composition according to claim5, wherein a composition ratio R₁ is greater than or equal to 0.4 andless than or equal to 2.5.
 7. The composition according to claim 1,wherein: GeO₂ has a concentration of about 50 mole percent to about 90mole percent; Tm₂O₃ has a concentration of about 0.001 mole percent toabout 2 mole percent; and Ga₂O₃, has a concentration of about 2 molepercent to about 40 mole percent.
 8. The composition according to claim3, comprising: GeO₂ having a concentration of about 65 mole percent toabout 75 mole percent; Tm₂O₃ having a concentration of about 0.05 molepercent to about 0.1 mole percent; Ga₂O₃, having a concentration ofabout 10 mole percent to about 18 mole percent; BaO having aconcentration of about 2 mole percent to about 7 mole percent; CaF₂having a concentration of about 0.1 mole percent to about 10 molepercent; CaO having a concentration of about 4 mole percent to about 10mole percent; K₂O having a concentration of about 2 mole percent toabout 6 mole percent; Rb₂O having a concentration of about 2 molepercent to about 4 mole percent; CeO₂ having a non-zero concentration ofless than about 2 mole percent; and Re₂O₃ having a concentration of lessthan about 10 mole percent, where RE is a rare earth element other thanthulium.
 9. The composition according to claim 1, further comprising:AX₂, having a concentration of about 0.5 mole percent to about 10 molepercent.
 10. The composition according to claim 1, further comprising:AX₂, having a concentration of about 2 mole percent.
 11. The compositionaccording to claim 10, wherein: GeO₂ has a concentration of about 60mole percent; Tm₂O₃ has a concentration of about 0.5 weight percent;Ga₂O₃ has a concentration of about 10 mole percent to about 18 molepercent; and wherein the composition further comprises BaO having aconcentration of about 18 mole percent to about 30 mole percent.
 12. Thecomposition according to claim 11, wherein a Tm³⁺ emissioncharacteristic of the composition is varied by selecting the compositionratio R₁.
 13. The composition according to claim 12, wherein R₁ isbetween 0.4 and 2.5.
 14. The composition according to claim 3, whereinthe composition has a composition ratio R₁ of about 0.8 to about 1.25.15. The composition according to claim 3, wherein the composition has acomposition ratio R₂ of about 1.8 to about 2.25, wherein R₂ is definedby the following relationship: R₂=(AO+AX₂)/(Z₂O+Z₂X₂).
 16. An opticalamplification device, comprising: a germanate glass material doped withTm³⁺ having a first surface configured to receive an optical signalhaving a wavelength of from about 1460 nm to about 1530 nm, and a secondsurface configured to output an amplified optical signal.
 17. Theoptical amplification device according to claim 16, wherein thegermanate glass material doped with Tm³⁺ comprises: GeO₂ having aconcentration of at least 20 mole percent; Tm₂O₃ having a concentrationof about 0.001 mole percent to about 2 mole percent; a compound selectedfrom the group consisting of Ga₂O₃, having a concentration of about 2mole percent to about 40 mole percent, SiO₂, having a non-zeroconcentration of less than about 40 mole percent, Ta₂O₅, having a nonzero concentration of less than about 15 mole percent, La₂O₃, having anon zero concentration of less than about 15 mole percent, Al₂O₃, havinga concentration of at least about 5 mole percent, and combinationsthereof, and an alkaline earth metal compound selected from the groupconsisting of MgO, CaO, SrO, BaO, BaF₂, MgF₂, CaF₂, SrF₂, BaCl₂, MgCl₂,CaCl₂, SrCl₂, BaBr₂, MgBr₂, CaBr₂, SrBr₂, and combinations thereof, andhaving a non-zero concentration of less than about 40 mole percent. 18.The optical amplification device according to claim 17, wherein thegermanate glass material doped with Tm³⁺ further comprises an alkalimetal compound selected from the group consisting of Li₂O, Na₂O, K₂O,Rb₂O, Cs₂O, Li₂F₂, Na₂F₂, K₂F₂, Rb₂F₂, Cs₂F₂, Li₂Cl₂, Na₂Cl₂, K₂Cl₂,Rb₂Cl₂, Cs₂Cl₂, Li₂Br₂, Na₂Br₂, K₂Br₂, Rb₂Br₂, Cs₂Br₂ and combinationsthereof, having a concentration of about 2 mole percent to about 6 molepercent.
 19. The optical amplification device according to claim 18,wherein the germanate glass material doped with Tm³⁺ has an emissionbandwidth that is varied based on the composition ratio R₁ being greaterthan or equal to 0.4 and less than or equal to 2.5, said emissionbandwidth extending from about 1400 nm to about 1540 nm, wherein saidemission bandwidth is measured as a full width at half maximum.
 20. Theoptical amplification device according to claim 16, wherein thegermanate host material comprises: GeO₂ having a concentration of about65 mole percent to about 75 mole percent; Tm₂O₃ having a concentrationof about 0.05 mole percent to about 0.1 mole percent; Ga₂O₃, having aconcentration of about 10 mole percent to about 18 mole percent; BaOhaving a concentration of about 2 mole percent to about 7 mole percent;CaF₂ having a concentration of about 0.1 mole percent to about 10 molepercent; CaO having a concentration of about 4 mole percent to about 10mole percent; K₂O having a concentration of about 2 mole percent toabout 6 mole percent; Rb₂O having a concentration of about 2 molepercent to about 4 mole percent; and CeO₂ having a non-zeroconcentration of less than about 2 mole percent.
 21. The opticalamplification device according to claim 18, further comprising: a pumpsource configured to output pump light, wherein a wavelength of the pumplight corresponds to an absorption characteristic of the germanate glassmaterial doped with Tm³⁺ to produce a population in a ³H₄ excited stateof Tm³⁺.
 22. The optical amplification device according to claim 16,wherein the germanate glass material doped with Tm³⁺ is a core for anoptical amplifying fiber.